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This is a heterogenous group of soft tissue tumors which have a common histologic appearance of abundant myxoid ground substance. This is composed of mucopolysaccharides, mainly hyaluronic acid.


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Thymoma Arising Within Cardiac Myxoma.

Miller DV, Tazelaar HD, Handy JR, Young DA, Hernandez JC.

From the *Division of Anatomic Pathology, Mayo Clinic, Rochester, MN; and the Departments of daggerUrology, double daggerCardiovascular Surgery, and section signPathology, Providence Portland Medical Center, Portland, OR.

Am J Surg Pathol. 2005 Sep;29(9):1208-1213. Abstract quote  

Hematopoietic, glandular, and mesenchymal elements can be found within cardiac myxomas; ectopic endocrine tissues and "thymic rests" have also rarely been described. Atrial tumors (one right and one left) from 2 patients (a 69-year-old man and a 77-year-old woman) were encountered among the atrial myxoma cases in one of the author's consultation files.

Both tumors were comprised of classic cardiac myxoma (with characteristic rings and syncytial chains of myxoma cells in a loose myxoid matrix) and cellular thymoma-like elements (characterized by a lobulated sheet-like growth of epithelioid spindle cells admixed with small lymphocytes punctuated by vessels with prominent perivascular spaces). Neither patient had evidence of thymoma elsewhere.

Immunophenotypically, the thymoma-like component reacted strongly with antibodies to keratins (AE1/AE3, Cam 5.2, wide spectrum, CK19, CK7) and CD57 and weakly with antibodies to CD31, CD34, and calretinin. This intermediate phenotypic expression of both epithelial and vascular antigens likely reflects the multipotential nature of the cells comprising this lesion.

The most likely explanation for this extremely unusual finding is neoplastic transformation of thymic rests within a myxoma.

5-7th decades
F slight predominance
Solitary slowing growing mass
Thigh or gluteal
5-10 cm
Rarely multiple
May be associated with underlying fibrous dysplasia, usually femur
Adjacent to joint, usually knee
Adults 4-5th decades
M predominance
5 cm or less
Pain common
Also called digital mucous cyst
Solitary painful nodule


Poorly circumscribed merging with adjacent skeletal muscle
Myxoid collections with paucity of blood vessels
No atypia or pleomorphism
Similar to intramuscular type
Deep dermal nodule similat to other myxomas
Hypercellularity is more common
The Cytopathology of Soft Tissue Mxyomas
Ganglia, Juxta-articular Myxoid Lesions, and Intramuscular Myxoma

Paul E. Wakely, Jr, etal.
Am J Clin Pathol 2005;123:858-865 Abstract quote

We studied the practicality of issuing a cytologic diagnosis of myxoma/juxta-articular myxoid lesion/ganglion (MJG) by reviewing all fine-needle aspiration (FNA) biopsy specimens of soft tissue masses in our files with diagnoses of myxoma, myxoid cyst, myxoid lesion, ganglion, or ganglion cyst. The control group was soft tissue aspirates with abundant myxoid stroma.

Of 39 cases with a cytologic diagnosis of soft tissue MJG, 15 had subsequent tissue biopsy or complete resection of the mass; 24 had clinicoradiologic follow-up. All cases except 1 (fat necrosis) were diagnosed correctly as benign myxoid lesions. We grouped MJG aspirates into 3 subtypes based on clinicoradiologic features: soft tissue ganglion/ganglion cyst (12 cases), juxta-articular myxoid lesion (16 cases), and intramuscular myxoma (11 cases). MJG aspirates showed few, subtle cytopathologic differences among subtypes. They characteristically had a viscous, gelatinous quality when expressed from the needle onto the glass slide. The typical smear contained a film of paucicellular, often finely granular, myxoid stroma that contained few cells, usually macrophages or bland spindle cells. Control group aspirates always contained cellular components that allowed distinction from MJGs.

The cytopathologic diagnosis of MJG lesions is accurate; FNA biopsy can be used to subtype MJGs into 3 categories when clinicoradiologic features are known.

Epidermal basaloid proliferation in cutaneous myxomas.

Mehregan DR, Thomas L, Thomas JE.

Pinkus Dermatopathology Laboratory, P.C., Monroe, and Wayne State University, School of Medicine, Detroit, MI, USA.

J Cutan Pathol. 2003 Sep;30(8):499-503 Abstract quote.  

BACKGROUND: Basaloid epidermal proliferations, which histologically resemble basal cell carcinoma, have been described overlying dermatofibromas. Several etiologies have been proposed. Cutaneous myxomas are also benign mesynchymal tumors.

PURPOSE: Basaloid proliferations have been noted overlying cutaneous myxomas. We have undertaken a study to attempt to differentiate whether these are basal cell carcinomas or benign basaloid proliferations.

METHODS: Thirty cases of cutaneous myxomas were included in this study. The lesions were stained with hematoxylin-eosin and alcian blue. Immunohistochemical staining for both epidermal growth factor receptor (EGF-r) and p53 protein was performed on the cutaneous myxomas with epidermal basaloid proliferation.

RESULTS: Of the 30 cases of cutaneous myxomas, nine were found to have an associated overlying basaloid proliferation. The basaloid proliferations were limited to the epidermis overlying the myxoid changes within the dermis. Mitotic figures were rare. Staining for p53 protein showed scattered positive staining in the basal cells in both the basaloid proliferations and adjacent epidermis. EGF-r showed positive staining of the overlying epidermis and basaloid proliferation in five cases.

CONCLUSIONS: We report basaloid proliferations overlying cutaneous myxomas and propose that these represent benign adnexal proliferations rather than superficial basal cell carcinoma and are analogous to the basaloid proliferations overlying dermatofibromas.

Intramuscular myxoma: a clinicopathologic study of 51 cases with emphasis on hypercellular and hypervascular variants.

Nielsen GP, O'Connell JX, Rosenberg AE.

James Homer Wright Pathology Laboratories, Massachusetts General Hospital, Harvard Medical School, Boston 02114, USA.

Am J Surg Pathol 1998 Oct;22(10):1222-7 Abstract quote

Intramuscular myxoma (IM) is a benign soft-tissue tumor that presents as a deeply seated mass confined to skeletal muscle. Surgical excision is virtually always curative. Recurrence, even after incomplete resection, is exceptional. Intramuscular myxoma is classically described as hypocellular and hypovascular, and is composed of cytologically bland stellate and bipolar fibroblasts separated by abundant extracellular myxoid matrix. What is underemphasized, however, is that IMs often show areas of increased cellularity and vascularity that can lead to a mistaken diagnosis of sarcoma, especially myxofibrosarcoma, low-grade fibromyxoid sarcoma, and myxoid liposarcoma.

In this report, we describe the clinicopathologic features of 51 IMs with special emphasis on those that exhibit these "hypercellular regions." The patients included 35 women and 16 men who ranged in age from 27 to 89 (mean 52) years. The tumors measured from 2 to 15 (average 5.6) cm and all had a gelatinous, lobulated cut surface. Histologically, they all demonstrated classic hypocellular, hypovascular regions. Thirty-eight tumors contained areas of relative increased cellularity that occupied from 10 to 80% of the tumor. These foci had increased numbers of cells, more prominent vascularity, and often increased collagen content. The hypercellular regions were not associated with cytologic atypia of the constituent cells, mitotic activity, or necrosis.

Follow-up information was available for 32 patients and ranged from 3 to 108 (average 30) months. No tumor recurred or metastasized. Areas of hypercellularity are common in IMs. Their recognition is important to avoid an erroneous diagnosis of sarcoma.


Prognostic Factors  

Henry JB. Clinical Diagnosis and Management by Laboratory Methods. Twentieth Edition. WB Saunders. 2001.
Rosai J. Ackerman's Surgical Pathology. Ninth Edition. Mosby 2004.
Sternberg S. Diagnostic Surgical Pathology. Fourth Edition. Lipincott Williams and Wilkins 2004.
Robbins Pathologic Basis of Disease. Seventh Edition. WB Saunders 2005.
DeMay RM. The Art and Science of Cytopathology. Volume 1 and 2. ASCP Press. 1996.
Weedon D. Weedon's Skin Pathology Second Edition. Churchill Livingstone. 2002
Fitzpatrick's Dermatology in General Medicine. 6th Edition. McGraw-Hill. 2003.
Weiss SW and Goldblum JR. Enzinger and Weiss's Soft Tissue Tumors. Fourth Edition. Mosby 2001.

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Last Updated September 16, 2005

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